Canine H3N8 Influenza Vaccine Partially Protects Against H3N2 Influenza in Mice

The canine H3N8 influenza vaccine provides partial cross-protection against canine H3N2 influenza in a mouse model, say the authors of a report recently published in Vaccine. The H3N8 vaccine did not prevent H3N2 infection, but it did limit viral replication in the lung. The authors conclude that the canine H3N8 vaccine may provide at least some protection against the canine H3N2 strain in dogs.

H3N8 influenza has been circulating among dogs in the United States since 1999, and a canine H3N8 vaccine was licensed in 2009. In 2015, a canine H3N2 influenza strain previously restricted to Asia caused a large US outbreak that began in the Chicago area. The canine H3N2 strain has since spread to other areas of the United States and has also caused illness in cats. The US Department of Agriculture has granted conditional licenses to two companies to produce vaccines against canine H3N2 influenza. Whether the H3N8 vaccine provides cross-protection against H3N2 in dogs is unknown.

The study authors investigated whether the canine H3N8 and H3N2 vaccines could produce antibodies against canine H3N2 influenza virus and prevent viral replication in mice. They injected mice with a canine H3N8 or H3N2 vaccine and repeated the dose 21 days later. Two weeks after the second vaccination, they collected serum samples and infected the mice with an intranasal dose of canine H3N2 virus. Three days later, they tested lung samples for virus isolates.

The authors report three main results:

• A hemagglutinin protein that is targeted by vaccines is structurally different between the canine H3N8 and H3N2 viruses. This represents an antigenic difference between the two strains.
• Despite the antigenic difference, antibodies produced by both vaccines reacted strongly to canine H3N2 virus in vitro.
• Canine H3N8 vaccination did not prevent H3N2 infection in vivo, but it did limit viral replication in the lung. Nearly all mice vaccinated with canine H3N8 vaccine had detectable H3N2 virus in the lungs after experimental infection. However, virus titers were significantly lower than in unvaccinated mice. Virus titers were also lower in H3N2-vaccinated mice, and fewer of these mice had detectable H3N2 virus in the lungs.

The authors conclude that the canine H3N8 vaccine elicits antibodies that can recognize the canine H3N2 virus. Although the canine H3N8 vaccine does not confer sterilizing immunity against H3N2, they say, the vaccine may protect dogs from developing serious disease. They recommend that in areas of the world where both strains are widespread, practitioners should consider using both vaccines.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC.